The present invention relates to a heat roller fixing device used in an electrophotographic image forming apparatus and, more particularly, to a heat roller fixing device having a curl correction mechanism.
In an electrophotographic image forming apparatus such as a printer, a toner image is transferred from a photosensitive drum to a recording medium by an image forming portion, and thereafter the transferred toner image is fixed to the recording medium by using a fixing device.
As a fixing device of this type, a heat roller fixing device is conventionally used which has a heat roller controlled at a predetermined temperature and a press roller having elasticity. A recording medium formed with a non-fixed toner image is passed through a fixing nip portion formed at the contact portion between these rollers, and is heated and pressed, thereby fixing the non-fixed toner image to the recording medium.
Since the heat roller fixing device heats the recording medium, a phenomenon in which the recording medium is curled to the heat roller side by heat occurs.
Due to this curl, the recording medium after fixing is not sometimes taken by a convey roller to cause jam of the recording medium on the convey path. This leads to the problem of degraded convey reliability.
In double-sided printing, curl that occurs after upper surface printing adversely affects an image on the leading end side in lower surface printing.
For this reason, when a heat roller fixing device is used, a mechanism that corrects the curl is required. A heat roller fixing device having a curl correction mechanism as follows is conventionally used accordingly.
For example, a method as indicated in Japanese Patent Laid-Open No. 8-314317 is employed in a fixing device in which a heater is brought into slidable contact with a fixing film and which has a press roller on the other side. A paper discharge lower guide, which regulates the convey direction of the recording medium immediately after the recording medium passes through a fixing nip portion is provided. The paper discharge lower guide is vertically moved to correct the curl of the recording medium that has passed through the fixing nip portion.
In a fixing device constituted by a heat roller and a press roller, as indicated in, e.g., Japanese Patent Laid-Open No. 8-290857, a discharge roller pair is arranged on the downstream side in the convey direction. A roller pair position variable means is arranged to change the position of the discharge roller pair with respect to the nip position of the heat roller and press roller in accordance with the temperature of the heat roller, thereby correcting the curl of the recording medium that has passed through the fixing nip portion.
In the prior arts, e.g., the prior arts employed in Japanese Patent Laid-Open Nos. 8-314317 and 8-290857, since curl correction of the recording medium is performed by correcting the recording medium convey path after the recording medium passes through the nip, a power to forcibly deform the recording medium cannot be obtained. If the curl is so large as to hinder conveyance of the recording medium, a sufficiently large correcting effect cannot be obtained.
In double-sided printing, unless the curl after upper surface printing is suppressed to be equal to or smaller than 1 mm, the curl adversely affects an image on the leading end side in lower surface printing. With the conventional arrangement, a sufficiently large correcting effect cannot be obtained.
With the conventional method, a drive mechanism that vertically moves the paper discharge guide or paper discharge roller, and a control mechanism that determines the vertical movement amount and designates it to the drive mechanism are required in order to correct the recording medium convey path.
Accordingly, the fixing device cannot be made compact, and the control operation of the system is complicated.